Lysosomal storage diseases (LSDs) are a large group of inherited enzyme deficiencies that produce fatal degenerative syndromes in children, with most having severe brain disease manifest as mental retardation. Neurogenetic diseases have lesions throughout the brain, thus it is thought that global correction will be needed. Vector mediated transfer of a normal cDNA of the gene can correct defective cells, but current vectors mediate relatively limited gene delivery in the brain. Nevertheless, CNS gene therapy may work in most LSDs because the enzyme is secreted from genetically corrected cells and taken up by neighboring cells; and the enzyme may be transported via axonal pathways to distal sites. To study approaches to gene therapy in a brain that is significantly larger than a rodent brain, we will use a cat model of human alpha- mannosidosis (AMD), caused by a mutation in the lysosomal alpha-mannosidase (LAMAN) gene. In the current grant period we found: 1) An AAV1 serotype vector increased the amount of total gene transfer and it expanded transduction to both white and gray matter areas. 2) A single surgical procedure injecting the vector into several tracks spaced around the brain produced enough normal LAMAN to reverse large areas of storage lesions. Clinically, the treated AMD cats showed promising improvements in the neurological syndrome, but not complete resolution. 3) Several magnetic resonance (MR) modalities were evaluated to non-invasively monitor disease pathology in the living animal brain. Magnetization transfer imaging (MTI) showed that significant improvements in the white matter could be measured in AMD cat brains treated by gene therapy. Diffusion-weighted imaging (Dw-MRI) and spectroscopy (1H-MRS) were able to detect disease in the gray matter areas, where most of the storage is seen. 4) Pathology studies demonstrated significant restoration of normal brain cell morphology in treated cats, in both gray and white matter, but some regions of uncorrected disease remained. To improve on these advances, we propose in the continuation grant to: 1) Evaluate a method to monitor vector gene expression in the living brain by PET. 2) Investigate alternate vector delivery approaches to achieve more complete correction of the CNS. 3) Study the ability of Dw-MRI and 1H-MRS to measure changes in the gray matter after treatment, using a 3T clinical magnet. 4) Study long-term correction (>1 year), the effects of starting treatment earlier in life, and evaluate functional correlates of disease correction in neuro-physiological studies. Together, these experiments in a mammalian brain that is significantly larger than a rodent brain provide a robust translational model to study the potential of AAV-mediated gene therapy to correct the global brain lesions in neurogenetic LSDs. The translational nature of these studies involves a team of experts from several fields, including neurology, MRI and PET imaging, gene vector development, molecular biology, neurophysiology, and pathology.